In conventional radiotherapy a source of radiation is used to irradiate a tumor, while shielding may be provided for the areas surrounding the tumor in order to minimize irradiation of the non-tumor areas. The shielding may be a simple milled block of dense material, where a void is milled in the negative of the tumor shape, thus permitting a beam of radiation largely shaped like the tumor to pass through. Such approaches are both simple and somewhat effective in concentrating radiation at the site of the tumor, while sparing surrounding tissue.
In proton therapy the use of a milled block is often employed. Proton therapy is distinct from photon therapy (e.g., x-ray) in that proton therapy uses accelerated particles, where the energy of the particle (e.g., 70 MeV-250 MeV) is set to determine how deeply into a body the particle will penetrate. The particle, when absorbed, transfers its energy to the absorbing tissue. This method of therapy enables more directed treatment of tumors, as compared to x-ray radiation (which penetrates through the body, transferring energy along its path).
Proton therapy can be broadly categorized into two modalities. The first is scattering proton therapy, wherein a radiation source is emitted over a wide field with portions of the radiation beam selectively blocked, in order to focus the radiation onto the target area (e.g., a tumor). The second is so-called “pencil beam” proton therapy, wherein a finely focused proton beam is scanned across a tumor. Somewhat akin to a cathode-ray tube television, a pencil beam proton therapy is magnetically directed in order to “paint” a tumor, delivering a very precise dose. This scanning is typically done without the presence of any collimator or MLC.
Scattering proton therapy may use multi-leaf collimators (MLCs), which are commonly used in x-ray radiotherapy machines. Typically, an MLC includes two sets of independently adjustable leaves. Each leaf is thick enough to attenuate or block completely a beam of radiation. The leaves can be positioned independently of one another to form an aperture in a shape like that of the area to be irradiated, so that only the targeted area is irradiated while surrounding areas are shielded.
A radiotherapy session may include radiation delivery from several (e.g., 2-5) dose angles. Compared with x-ray radiation, the leaves of the MLC need to be larger (e.g., thicker) in order to direct, or block, a proton beam. This leads to increased weight of a proton beam MLC compared to an MLC used exclusively for x-ray radiotherapy, with corresponding greater difficulty in manipulating the collimators during treatment (e.g., swapping out collimators, or changing angles for multiple doses). In particular, the weight of a MLC designed for proton therapy restricts its use on a gantry. These proton therapy designs (e.g., those having thick, dense MLC leaves) also cost more and have greater inertia due to the MLC leaf weight.